1. Technical Field of the Invention
The present invention relates to road motor vehicles using electricity for their propulsion.
2. The Related Art
For example, series hybrid vehicles and parallel hybrid vehicles are known. In vehicles with a so-called series hybrid drivetrain, an engine drives an alternator which converts the mechanical energy available at the shaft of the engine into electrical energy. This electrical energy supplies one or more electric traction motors mechanically connected to the driving wheels of the vehicle. The engine is not mechanically connected to the wheels. In vehicles with a so-called parallel hybrid drivetrain, an engine and an electric motor are both mechanically connected to the driving wheels; the torques which they deliver can be added together to drive the driving wheels.
In both cases, it is well known to install electric accumulators, especially where it is desired to be able to drive the vehicle in purely an electric mode, with the engine switched off. It is also well known to recover at least part of the energy on braking a vehicle, by operating the electric traction motor as a generator, and thus converting the kinetic energy into electrical energy which is stored, rather than dissipating as heat.
However, the use of electric accumulators presents various problems. Such a device has a high mass compared with the amount of electrical energy stored. For example, a lead-acid battery can store about 20 Wh/kg. This considerably increases the mass of a vehicle. It is also a source of a waste of energy during acceleration, and even during braking, since the efficiency of the conversions of kinetic energy into electrical energy and of electrical energy into kinetic energy cannot be 100%. Furthermore, the problems with the dynamic behavior of a vehicle are all the more crucial if the vehicle is heavy. In addition, the known batteries do not accept sufficiently intense recharging currents to be able to rapidly recover a lot of braking energy. And it has been found that the greater the desire to recover the energy on braking, the greater is the mass of batteries installed in the vehicle, this in itself being an energy consumer during the acceleration phases. Of course, the lower the efficiencies in the energy conversions, the less is recovered on braking of the energy which has had to be expended to accelerate this excess mass due to the batteries.
The object of the present invention is to design a drivetrain having a device in which the electrical energy can be stored during braking. The problem which arises, therefore, to be able to recover and store sufficient energy to improve the overall efficiency significantly, that is to say, to reduce the fuel consumption for a given duty.